This invention relates to alternating current power supplies and more particularly to such power supplies which employ a cycloconverter for transforming power from a direct current source into a pseudo-sine wave power output.
Cycloconverter power supply circuits in which positive and negative banks of switching elements are gated to convert the alternating current output of an inverter circuit at one frequency into a pseudo-sine wave output of a lower frequency are well known in the art. Such a power supply circuit is frequently employed to convert direct current power from a battery source to an alternating current output which very closely approximates a sine wave. Generally, as is known, the magnitude of the generated inverter output voltage is directly proportional to the magnitude of the battery source voltage applied thereto and any variation in the source voltage is reflected in the inverting alternating output voltage and, hence, in the ultimate cycloconverter output. Discharge of the battery, for example, with its resulting voltage reduction as well as changes in load, will also cause a change in the alternating current output power of the cycloconverter. Although in many applications of a cycloconverter power supply variations in the magnitude of the voltage output are not critical, in others, the alternating current converter output is held to tolerances less than the allowable battery source variation. Thus, for example, if the cycloconverter output is employed to drive an electric motor, variations, in the applied voltage may not only cause fluctuations in the power delivered by the motor but could also result in its damage. The cycloconverter circuit of this invention is thus chiefly directed to the problem of maintaining a substantially constant output power magnitude despite variations in the battery source voltage.
A feature of the cycloconverter power supply of the invention is directed to the equally important objective of maintaining to a minimum its cost, specifically, the cost of the inverter transistors. Normally, the cost of a transistor is directly proportional to its current conducting ability. Accordingly, to the extent that the current carrying requirements imposed on a transistor can be held to a minimum, to that extent its cost can be held down. A prior art power supply arrangement in which a voltage compensation function is performed directed to broadly similar ends is described, for example, in U.S. Pat. No. 3,491,282 of T. M. Heinrich et al., issued Jan. 20, 1970. The power supply circuit of the present invention is also a departure from the regulated cycloconverter of U.S. Pat. No. 3,743,919 of D. W. Bingley, issued July 3, 1973.